The Germans have specifically ruled out doing East Prussia again, for the same reason a recovering alcoholic won’t touch a drop of wine. Which leaves two options: Králiveč, the Czech exclave with a beer pipeline to the mainland*, or the more mundane option of partitioning it somehow between Poland and Lithuania.
* which should probably be named in honour of Immanuel Kant, a famous local reputed to have been a real pissant who was very rarely stable
Germans are still suffering from having to integrate another failed quasi-soviet state more than 30 years later, so you couldn't pay them enough for taking Königsberg back.
Though having instantly given citizenship to thousands of Russians on the grounds that their ancestors around the time of Catherine the Great were German, and ending up with a monolingually Russophone voting bloc that gets its news from Russian state TV, did them no favours either. Jus sanguinis: not even once.
However, it would be somewhat entertaining to see the AfD melting down over getting it back. On the one hand, you have some of the old territory back, but on the other hand, there's so many foreigners getting your nationality now…
It implies that Kaliningrad is left undefended and up for the taking and that the surrounding countries are all looking at each other wondering which one of them is going to make a move on it.
Germany and Czechia are not surrounding countries but Kaliningrad (then named Královec) was founded by Czech king Přemysl Otakar II. at the height of the Bohemian empire, and was later part of Prussia (basically Germany in the 1800s).
I don't think many Germans are joking about annexing Kaliningrad because of the WWII stigma but Czech redditors regularly invade r/Kaliningrad only to get promptly banned.
The thing about Kaliningrad is that Russia would have no way of reinforcing it. If Ukraine occupied it, they could only move there via boat, since it is surrounded by NATO countries. That makes it incredibly easy to defend, especially if they move some sea and air drones over there.